1. Field of the Invention
The present invention relates to imidazolium salts. More particularly, the present invention relates to methods of producing imidazolium salts.
2. General Background of the Invention
N-heterocyclic carbenes1, so called “phosphine-mimics”, have attracted considerable attention as possible alternatives for the widely used phosphine2 ligands in homogeneous catalysis.34
The primary advantage of these ligands appears to be that they do not easily dissociate from the metal center, and as a result an excess of the ligand is not required in order to prevent aggregation of the catalyst usually affording the bulk metal5. The use of these ligands in palladium-catalyzed Heck6, Suzuki-Miyaura7, Stille8, Sonogashira9, aryl amination10, telomerization of butadiene11, Hiyama12, Kumada-Corriu reactions13 14 15, rhodium-assisted hydrosilylation16, iridium-mediated olefin hydrogenation17 and ruthenium-mediated olefin metathesis7,8 has opened new opportunities in catalysis. The synthesis of the NHC generally involves the deprotonation of the imidazolium precursor by action of a base.
The synthesis of the imidazolium salts as reported by Arduengo18 involves a condensation reaction with concomitant azeotropic removal of water at elevated temperatures.
This protocol is not general for aryl bearing imidazolium and sterically congested imidazolium salts (bulky groups on nitrogen) often fails or leads to low yields of product.
The previously reported synthetic procedures for the preparation of imidazolium salts necessitated high temperature, dry solvents and handling under inert atmosphere.18 An alternative method employed molecular sieves to absorb the water generated in the condensation reaction. It also required heating for long reaction times.2 It should also be noted that these reaction times would have to be significantly longer if these protocols were performed on very large scale. As a result of prolonged heating, decomposition occurs and tars are generated which render product isolation difficult and isolated yield lower than optimum. As some imidazolium salts are not at all amenable to assembly using the “one-pot” protocol, we have explored the possibility of performing the condensation reactions in a two-step procedure.
The following U.S. patents are incorporated herein by reference: U.S. Pat. Nos. 5,077,414; 5,182,405; and 6,316,380